The present invention relates to a jitter measurement apparatus and a jitter measurement method.
A Time Interval Analyzer or an oscilloscope has conventionally been used in a period jitter measurement. Each of those methods is called a Zero-crossing Method. As shown in FIG. 1, for example, a clock signal (a signal under measurement) x(t) from a PLL (Phase-Locked Loop) under test 11 is supplied to a time interval analyzer 12. Regarding a signal under measurement x(t), a next rising edge following one rising edge fluctuates against the preceding rising edge as indicated by dotted lines. That is, a time interval Tp between two rising edges, namely a period fluctuates. In the Zero-crossing Method, a time interval between zero-crossings (period) is measured, a relative fluctuation of period is measured by a histogram analysis, and its histogram is displayed as shown in FIG. 2. A time interval analyzer is described in, for example, xe2x80x9cPhase Digitizing Sharpens Timing Measurementsxe2x80x9d by D. Chu, IEEE Spectrum, pp.28-32, 1988 and xe2x80x9cA Method of Serial Data Jitter Analysis Using One-Shot Time Interval Measurementsxe2x80x9d by J. Wilstrup, Proceedings of IEEE International Test Conference, pp.819-823, 1998.
On the other hand, Tektronix, Inc. and LeCroy co. have recently been providing digital oscilloscopes each being able to measure a jitter using an interpolation method. In this jitter measurement method using the interpolation method (interpolation-based jitter measurement method), an interval between data having signal values close to a zero-crossing out of measured data of a sampled signal under measurement is interpolated to estimate a timing of zero-crossing. That is, a time interval between zero-crossings (period) is estimated with a small error to measure a relative fluctuation of period.
That is, as shown in FIG. 3, a signal under measurement x(t) from the PLL under test 11 is inputted to a digital oscilloscope 14. In the digital oscilloscope 14, as shown in FIG. 4, the inputted signal under measurement x(t) is converted into a digital signal data sequence by an analog-to-digital converter 15. A data-interpolation is applied to an interval between data having signal values close to a zero-crossing in the digital data sequence by an interpolator 16. With respect to the data-interpolated digital data sequence, a time interval between zero-crossings is measured by a period estimator 17. A histogram of the measured values is displayed by a histogram estimator 18, and a root-mean-square value and a peak-to-peak value of fluctuations of the measured time intervals are obtained by an RMS and Peak-to-Peak Detector 19. For example, in the case in which a signal under measurement x(t) has a waveform shown in FIG. 5A, its period jitters are measured as shown in FIG. 5B.
On the contrary, we have proposed a xcex94xcfx86 method for measuring a jitter by obtaining a variable component (phase noise) of an instantaneous phase of a signal under measurement. This xcex94xcfx86 method is characterized in that an instantaneous phase of a signal under measurement is estimated using an analytic signal theory. FIG. 6 shows a processing block diagram of the xcex94xcfx86 method. An input signal is transformed into a complex analytic signal by a Hilbert pair generator 21. An instantaneous phase of an input signal is obtained from the complex analytic signal by an instantaneous phase estimator 22. A linear phase component is moved from the instantaneous phase by a linear trend remover 23 to extract a phase noise. With respect to this phase noise, a sample value closest to a zero-crossing point in a real part of the complex analytic signal is extracted by a zero-crossing resampler 24 to obtain a timing jitter sequence. A peak-to-peak value of the output of the zero-crossing resampler 24 is obtained by a xcex94xcfx86PP detector 25 as a peak-to-peak jitter xcex94xcfx86PP of the input signal. In addition, a root-mean-square value of the output of the zero-crossing resampler 24 is obtained by a xcex94xcfx86RMS detector 26 as a root-mean-square value xcex94xcfx86RMS of jitter of the input signal. Furthermore, a histogram of each sample value of the resampler 24 is displayed and estimated by a histogram estimator 27. This xcex94xcfx86 method is described in, for example, xe2x80x9cExtraction of Peak-to-Peak and RMS Sinusoidal Jitter Using an Analytic Signal Methodxe2x80x9d by T. J. Yamaguchi, M. Soma, M. Ishida, T. Watanabe, and T. Ohlmi, Proceedings of 18th IEEE VLSI Test Symposium, pp. 395-402, 2000.
In the jitter measurement method by the time interval analyzer method, a time interval between zero-crossings is measured. Therefore a correct measurement can be performed. However, since there is, in this jitter measurement method, a dead-time when no measurement can be performed after one period measurement, there is a problem that it takes a long time to acquire a number of data that are required for a histogram analysis. In addition, in an interpolation-based jitter measurement method in which a wide-band oscilloscope and an interpolation method are combined, there is a problem that a jitter is overestimated (overestimation). That is, there is no compatibility in measured jitter values between this jitter measurement method and the time interval analyzer method. For example, a result of jitter measurement using a time interval analyzer for a clock signal of 400 MHz is shown in FIG. 7A, and a result of jitter measurement using an interpolation-based jitter measurement method for the same clock signal is shown in FIG. 7B.
Those measured results are, a measured value by the time interval analyzer 7.72 ps (RMS) vs. a measured value by the interpolation-based jitter measurement method 8.47 ps (RMS), and the latter is larger, i.e., the measured value by interpolation-based jitter measurement method has overestimated the jitter value. In addition, the interpolation-based jitter measurement method cannot correctly estimate a Gaussian distribution having single peak.
It is an object of the present invention to provide a jitter measurement apparatus and its method that can estimate a jitter value having compatibility, similarly to the xcex94xcfx86 method, with a conventional time interval analyzer method, i.e., a correct jitter value in a shorter time period.
The jitter measurement apparatus according to the present invention comprises: analytic signal transformation means for transforming a signal under measurement into a complex analytic signal; instantaneous phase estimation means for obtaining an instantaneous phase of the signal under measurement from the complex analytic signal transformed by the analytic signal transformation means; linear instantaneous phase estimation means for obtaining a linear instantaneous phase of an ideal signal that does not contain a jitter by obtaining a least mean square line of the estimated instantaneous phase; zero-crossing timing estimation means for obtaining a zero-crossing timing of the signal under measurement using an interpolation method or an inverse interpolation method; timing jitter estimation means for calculating a difference between an instantaneous phase value of the signal under measurement and a linear instantaneous phase value of the ideal signal at the zero-crossing timing estimated by the zero-crossing timing estimation means to obtain a timing jitter sequence; and a jitter detection means to which the timing jitter sequence is supplied for obtaining a jitter of the signal under measurement.
In addition, it is desirable that the jitter measurement apparatus further comprises period jitter estimation means to which the timing jitter sequence is inputted for calculating its differential sequence and for outputting a period jitter sequence of the signal under measurement to supply it to the jitter detection means.
In addition, it is desirable that the jitter measurement apparatus further comprises cycle-to-cycle period jitter estimation means to which the period jitter sequence is inputted for calculating its differential sequence and for outputting a cycle-to-cycle period jitter sequence to supply it to the jitter detection means.
In addition, it is desirable that the zero-crossing timing estimation means comprises: linear instantaneous phase data interpolation means to which the linear instantaneous phase data are supplied for interpolating linear instantaneous phase data between a plurality of linear instantaneous phase data around a predetermined phase value in the linear instantaneous phase data; zero-crossing data determination means for determining a data closest to the predetermined value in the data-interpolated linear instantaneous phase data; and timing estimation means for estimating a timing of the determined data.
Alternatively, it is desirable that the zero-crossing timing estimation means is means to which the linear instantaneous phase data are supplied for estimating a zero-crossing timing by the inverse interpolation method form a plurality of linear instantaneous phase data around a predetermined phase value in the linear instantaneous phase data and for outputting the zero-crossing timing.
Or, it is desirable that the zero-crossing timing estimation means comprises: instantaneous phase data interpolation means to which the instantaneous phase data are supplied for interpolating instantaneous phase data between a plurality of instantaneous phase data around a predetermined phase value in the instantaneous phase data; zero-crossing data determination means for determining a data closest to the predetermined phase value in the data-interpolated instantaneous phase data; and timing estimation means for estimating a timing of the determined data.
Or, in the jitter measurement apparatus, it is desirable that the zero-crossing timing estimation means is means to which the instantaneous phase data are supplied for estimating a zero-crossing timing by the inverse interpolation method from a plurality of instantaneous phase data around a predetermined phase value in the instantaneous phase data and for outputting the zero-crossing timing.
Or, it is desirable that the zero-crossing timing estimation means comprises: waveform data interpolation means to which real parts (real signal waveform data) of the complex analytic signal are supplied for interpolating waveform data between a plurality of waveform data around a zero-crossing in the real signal waveform data; zero-crossing data determination means for determining a waveform data closest to the zero-crossing in the data-interpolated real signal waveform data; and timing estimation means for estimating a timing of the determined data.
Or, it is desirable that the zero-crossing timing estimation means is means to which real parts (real signal waveform data) of the complex analytic signal are supplied for estimating a zero-crossing timing by the inverse interpolation method from a plurality of waveform data around zero-crossing in the real signal waveform data and for outputting the zero-crossing timing.
In addition, it is desirable that the analytic signal transformation means comprises: band-pass filtering means to which the signal under measurement is supplied for extracting only components around a fundamental frequency from the signal under measurement to limit the bandwidth of the signal under measurement; and Hilbert transformation means for Hilbert-transforming an output signal of the band-pass filtering means to generate a Hilbert pair of the input signal.
Alternatively, it is desirable that the analytic signal transformation means comprises: time domain to frequency domain transformation means to which the signal under measurement is supplied for transforming the signal under measurement into a both-sided spectrum signal in frequency domain; bandwidth limiting means for extracting only components around a positive fundamental frequency in the both-sided spectrum signal; and frequency domain to time domain transformation means for inverse-transforming an output of the bandwidth limiting means into a signal in time domain.
Or, it is desirable that the analytic signal transformation means comprises: a buffer memory to which the signal under measurement is supplied for storing therein the signal under measurement; means for taking out the signal in the sequential order from the buffer memory such that the signal being taken out is partially overlapped with the signal taken out just before; means for multiplying each taken out partial signal by a window function; means for transforming each partial signal multiplied by the window function into a both-sided spectrum signal in frequency domain; bandwidth limiting means for extracting only components around a positive fundamental frequency of the signal under measurement from the both-sided spectrum signal transformed in frequency domain; means for inverse-transforming an output of the bandwidth limiting means into a signal in time domain; and means for multiplying the signal transformed in time domain by an inverse number of the window function to obtain a band-limited analytic signal.
In addition, it is desirable that the jitter measurement apparatus further comprises AD conversion means for inputting therein the signal under measurement for digitizing and converting an analog signal into a digital signal to input the digital signal to the analytic signal transformation means.
In addition, it is desirable that the jitter measurement apparatus further comprises waveform clipping means to which the signal under measurement is inputted for removing amplitude modulation components of the signal under measurement to supply a signal having only phase modulation components of the signal under measurement to the analytic signal transformation means.
In addition, it is desirable that the jitter detection means is peak-to-peak detection means for obtaining a difference between the maximum value and the minimum value of the supplied jitter sequence.
In addition, it is desirable that the jitter detection means is RMS detection means for obtaining a root mean square value (RMS value) of the supplied jitter sequence.
In addition, it is desirable that the jitter detection means is histogram estimation means for obtaining a histogram of the supplied jitter sequence.
A jitter measurement method according to the present invention has the steps of: transforming a signal under measurement into a complex analytic signal; estimating an instantaneous phase of the signal under measurement from the complex analytic signal; estimating a linear instantaneous phase of an ideal signal that does not contain a jitter by obtaining a least mean square line of the instantaneous phase; estimating a zero-crossing timing of the signal under measurement using the interpolation method or the inverse interpolation method; calculating a difference between an instantaneous phase value of the signal under measurement and a linear instantaneous phase value of the ideal signal at the zero-crossing timing to estimate a timing jitter sequence; and obtaining a jitter of the signal under measurement from the jitter sequence.
In addition, it is desirable that the jitter measurement method has the step of providing the timing jitter sequence as an input for calculating its differential sequence and for outputting a period jitter sequence of the signal under measurement.
In addition, it is desirable that the jitter measurement method has the steps of: providing the timing jitter sequence as an input for calculating its differential sequence and for outputting a period jitter sequence of the signal under measurement; and providing the period jitter sequence as an input for calculating its differential sequence and for outputting a cycle-to-cycle period jitter sequence.
In addition, it is desirable that the step of estimating a zero-crossing timing has the steps of: interpolating linear instantaneous phase data between a plurality of linear instantaneous phase data around a predetermined phase value in the linear instantaneous phase data; determining a data closest to the predetermined phase value in the data-interpolated linear instantaneous phase data; and estimating a timing of the determined data.
Alternatively, it is desirable that the step of estimating the zero-crossing timing is a step of estimating a zero-crossing timing by the inverse interpolation method from a plurality of instantaneous phase data around a predetermined phase value in the instantaneous phase data.
Or, it is desirable that the step of estimating a zero-crossing timing comprises the steps of: interpolating instantaneous phase data between a plurality of instantaneous phase data around a predetermined phase value in the instantaneous phase data; determining a data closest to the predetermined phase value in the data-interpolated instantaneous phase data; and estimating a timing of the determined data.
Or, it is desirable that the step of estimating a zero-crossing timing is a step of estimating a zero-crossing timing by the inverse interpolation method from a plurality of instantaneous phase data around a predetermined phase data in the instantaneous phase data.
Or, it is desirable that the step of estimating a zero-crossing timing comprises the steps of: interpolating waveform data between a plurality of waveform data around a zero-crossing in the real part (real signal waveform data) of the analytic signal; determining a waveform data closest to the zero-crossing in the data-interpolated signal waveform data; and estimating a timing of the determined data.
Or, it is desirable that the step of estimating a zero-crossing timing is a step of estimating a zero-crossing timing by an inverse interpolation method from a plurality of waveform data around a zero-crossing in the real part (real signal waveform data) of the analytic signal.
In addition, it is desirable that the step of interpolating data around the predetermined value is a step of data-interpolating using a polynomial interpolation method.
Attentively, it is desirable that the step of interpolating data around the predetermined value is a step of data-interpolating using a cubic spline interpolation method.
Or, it is desirable that the step of estimating the zero-crossing is a step of using an inverse linear interpolation method.
In addition, it is desirable that the step of transforming the signal under measurement into an analytic signal comprises the steps of: extracting only components around a fundamental frequency from the signal under measurement to limit the bandwidth of the signal under measurement; and Hilbert-transforming the band-limited signal to generate a Hilbert pair of the input signal.
Alternatively, it is desirable that the step of transforming the signal under measurement into an analytic signal comprises the steps of: transforming the signal under measurement into a both-sided spectrum signal in frequency domain; extracting only components around a positive fundamental frequency in the both-sided spectrum signal; and inverse-transforming the extracted components around the positive fundamental frequency into a signal in time domain.
Or, it is desirable that the step of transforming the signal under measurement into an analytic signal comprises the steps of: storing the signal under measurement in a buffer memory; taking out the signal in the sequential order from the buffer memory such that the signal being taken out is partially overlapped with the signal taken out just before; multiplying each taken out partial signal by a window function; transforming each partial signal multiplied by the window function into a both-sided spectrum signal in frequency domain; extracting only components around a positive fundamental frequency of the signal under measurement from the both-sided spectrum signal transformed in frequency domain; inverse-transforming the extracted spectrum signal having components around the positive fundamental frequency into a signal in time domain; and multiplying the signal transformed in time domain by an inverse number of the window function to obtain a band-limited analytic signal.
In addition, it is desirable that the jitter measurement method has a step of performing a waveform-clipping of the signal under measurement to remove amplitude components of the signal under measurement and to extract a signal containing only phase modulation components of the signal under measurement, and for transferring the signal to the step of transforming the signal under measurement into an analytic signal.
In addition, it is desirable that the step of obtaining a jitter is a step of obtaining a difference between the maximum value and the minimum value of the timing jitter sequence to calculate a peak-to-peak value.
Alternatively, it is desirable that the step of obtaining a jitter is a step of obtaining a root mean square value of the timing jitter sequence to calculate an RMS value.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a histogram data of the timing jitter sequence.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a difference between the maximum value and the minimum value of the period jitter sequence to calculate a peak-to-peak value.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a root mean square value of the period jitter sequence to calculate an RMS value.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a histogram data of the period jitter sequence.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a difference between the maximum value and the minimum value of the cycle-to-cycle period jitter sequence to calculate a peak-to-peak value.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a root mean square value of the cycle-to-cycle period jitter sequence to calculate an RMS value.
Or, it is desirable that the step of obtaining a jitter is a step of obtaining a histogram data of the cycle-to-cycle period jitter sequence.
Or, it is desirable that a part or all of the peak-to-peak values, RMS values, and histogram data are obtained.